The present invention relates to a process of preparing a gas composition comprising N2, CO2, minor parts of NOx, VOC (volatile organic compounds) and about 0-2.0 mole % O2 from an oxygen containing exhaust gas and use thereof.
It is well-known to inject CO2 in submarine formations and oil-containing fields to reduce the discharge of CO2 to the atmosphere. Injection of CO2 into oil-containing fields simultaneously increases the recovery of oil from said oil-containing field. Further it is also known to produce electrical power from fossil fuel. Increasing the quantity of oil recovery by water injection, nitrogen and CO2 separated from a oil/gas mixture is also known in the art.
Large quantities of gas are required to recover oil in an oil-containing field by injection of a gas CO2 can be obtained by separating CO2 either from the feed gas or from combustion flue gas e.g. from a gas power plant. Large amounts of flue gas are required when only the CO2 in the combustion flue gas is used for injection in oil-containing fields to recover oil, since the typical content of CO2 is in the magnitude of 3-10 mole %. The remainder of the combustible constituents are conveyed forward for combustion for the production of electrical power by purification of the feed gas (e.g. upstream to a gas power plant). The remainder of the constituents are conveyed to the atmosphere by purification of the combustion gas (e.g. downstream to a gas power plant). Due to the large gas quantities that are required for injection in oil-containing fields to increase the recovery of oil, an inert gas composition consisting mainly of CO2 can be obtained by the separation of CO2 from a fuel gas or a flue gas in a gas fired power plant where the production of large amounts of electrical power is involved. Said production of electrical power may cause problems concerning sales of electrical power and in particular cases transmission capacity of electrical power.
The object of the present invention is to provide a process of preparing a gas composition suitable for injection in an oil-containing field in such a way to increase the degree of recovery and simultaneously reduce the discharge of CO2 to the atmosphere. However, injection of a gas composition containing large amounts of oxygen will result in a degraded submarine formation, which is not desirable. The present invention thus provides a process for the production of a gas composition comprising N2, CO2, minor parts of NOx, VOC (volatile organic compounds) and about 0-2.0 mole % O2 from an oxygen containing exhaust gas, wherein a stream of a exhaust gas from the combustion of air and natural gas in a gas turbine containing 18% by weight O2 or less, is combusted with natural gas, whereupon the desired gas composition is discharged as a flue gas. The content of O2 in the exhaust gas from the combustion of air and natural gas in a gas turbine is in the magnitude of 12-18% O2. The gas composition obtained preferably contains about 0 to 1.0 mole % O2, and more preferably about 0.1-0.5 mole % O2. The present invention may utilise any exhaust gas from a combustion process, but a preferred embodiment of the invention is the production of an inert gas composition from the combustion products from a gas turbine plant. The present invention also comprises a process of recovering oil from submarine formations wherein a gas composition as mentioned above, comprising N2, CO2, minor parts of NOx and VOC (volatile organic compounds), and O2 is further compressed and injected into said submarine formation. Further, the use of a gas composition comprising N2, CO2, minor parts of NOx, VOC (volatile organic compounds) and about 0-2.0 mole % O2 from an oxygen containing flue gas for injection in an oil-containing field for the recovery of oil is also disclosed by the present invention.
The present Invention thus comprises a process of preparing a gas composition where said gas composition contains from 0-1.0 mole % O2, more preferably from 0.1-0.5 mole % O2. Previous problems concerning the degradation of oil-containing fields are reduced by said amounts of O2 which also are to be considered as minor. The process comprises eliminating molecular oxygen of the exhaust gases by charging a metered amount of fuel sufficient to combust the remaining amounts of oxygen almost completely (stoichiometric or reaction equivalent combustion) in a steam boiler. Steam is generated in the steam boiler, which is utilised for operating a vapor turbine which further operates an electrical generator.
One of the objects of preparing an inert gas composition as described herein is that said process also addresses an energy economy aspect. Large amounts of an inert gas composition are also provided by preparing a gas composition from any exhaust gas of a combustion process, not only using CO2 as an inert gas, but wherein also the remaining components, except for O2 are used. The amount of CO2 prepared from natural gas constitutes only 8 to 10% of the total gas amounts of the present invention. Combustion of exhaust gas from, e.g., a gas turbine plant is produced in stoichiometric amounts, whereby the remaining amounts of O2 are mainly converted to CO2. According to the present invention, a smaller gas turbine plant is thus provided including large amounts of exhaust gas per generated amounts of electric power for the preparation of a desired amount of an inert gas composition. The injection of an inert gas in an oil-containing field provides for the reduction of discharge to the atmosphere of CO2 from the power plant.
The present process can be preformed in a processing plant onshore or on an offshore installation such as a platform or a barge. The production of electric power according to the present invention on a offshore installation or an oil drilling platform, will provide a contribution to or a coverage of the power demand. A further advantage of such an offshore installation at oil-containing fields is the possibility of moving it from one field to another.
The temperature of the flue gases after combustion is decided by the exploitation ratio and the dimensioning of the steam boiler. The flue gases are dried and compressed to the desired pressure dependent on the character of the reservoir prior to injection.